I get my Mordant at a bulk food store. Alum (a white powder) is used for pickling and can be found readily. Mine looks like this. There is a food grade and the kind you can by from www.dharmatrading.com which is stronger but both work the same in my book. Cream of Tartar is generally used to thicken stock and gravy's and can be found all over - its a finer powder than Alum. 8 Tbs = 4 oz on my scale and 2 Tbs of Cream of Tartar = 1oz - You need that amount to mordant 1 pound of dry material (wool/silk etc). I mordant my materials before I dye. You put the required amount of mordant in 4 gallons of water. Heat this till it boils and then take it off the heat. Submerge the materials (do not boil wool or silk as it will loose it luster or felt!). and let it cool overnight. You can use it after one hour if you are short on time. You also can rinse your materials, dry them and store them for later. I usually do a marathon of mordanting and then I am ready to go later.
Take your onion skins and tie them in netting or cheese cloth and submerge them in a large pot of water.
Let this simmer for a few hours. Talk out the onions and pre-wet the fabric/wool/yarn with very warm water and then squeeze that out and place your item in the dye pot. I have an old wooden spoon I no longer use for food to push it under the dye. The photo shows the material in with the onion skins - you can do that too. You want to stir it every once in a while to move air bubbles around - but be gentle.
To test the color bring out a piece and squeeze out the liquid to look. Wet is always darker than dry. Rinse in cold water till it runs clear and hang to dry. Hang it out smooth so any left over color doesn't pool in a crease.I have found that onion skins dye unevenly and requires more stirring. However I have pulled a silk out with lovely variegation in color. Osage orange which is a tree bark you can get as saw dust or shavings produces an even dye color (yellow) on silks and cloth. These yarns are all onion skins - amazing you can get such different colors from different types of wool yarn.
I have a few more images but I have having trouble listing them - I will add them later. Good luck dyeing!